Cemented chromium carbide containing small additions of phosphorus



3,445,203 CEMENTED CHROMIUM CARBIDE CONTAINING SMALL ADDITIONS OFPHOSPHORUS William A. Powell, Detroit, Mich., assignor to GeneralElectric Company, a corporation of New York No Drawing. Filed May 1,1968, Ser. No. 725,929 Int. Cl. B22f 3/12 US. Cl. 29182.8 6 ClaimsABSTRACT OF THE DISCLOSURE Cemented carbide alloys containing chromiumcarbide and a nickel binder are prepared from chromium carbide andnickel powder to which has been added from a trace to about 0.4% byweight of phosphorus. The addition of phosphorus to the cemented carbidecomposition has been found to considerably enhance the resultingtransverse rupture strength of the alloy without compromise of theremaining desirable properties of the alloy, including high hardness,general corrosion and oxidation resistance and high thermal coefiicientof expansion.

This invention relates to cemented chromium carbide alloys havingconsiderably enhanced strength and to a process for preparing suchalloys.

Cemented carbides are well known for their unique combination ofhardness, strength and abrasion resistance and are accordinglyextensively used in industry as cutting tools, drawing dies and wearparts. Presently, the most extensively used cemented carbide alloys arecomposed of tungsten carbide and cobalt because of their unexcelledcombination of hardness and strength or toughness. However, the tungstencarbide-cobalt alloys have certain deficiencies, such as theirrelatively low oxidation resistance and their susceptibility tocorrosion in media such as aqueous acids. Moreover, they have a very lowcoeflicient of thermal expansion relative to that of steel. Thus, forexample, the high hardness of the tungsten carbide-cobalt alloys makesthem attractive materials for gauge blocks but their thermal expansioncharacteristics differ appreciably from that of steel, the materialagainst which the gauges are most frequently applied. Tungstencontainingalloys are also of high density and cobalt-containing alloys aregenerally magnetic, characteristics which are a disadvantage in certainapplications.

Other alloy systems, such as titanium carbide and tantalum carbide, havesimilar shortcomings. Improved corrosion resistance can be achieved byusing nickel and nickel alloys as a binder in tungsten carbide alloys,but such alloys still have relatively poor oxidation resistance, displaycomparatively low thermal expansion characteristics and are ofrelatively high density.

Chromium carbide-nickel cemented carbide alloys possess a number of verydesirable properties. They are characterized by a higher coefiicient ofthermal expansion than the other cemented carbidescloser to that ofsteela low density, they are nonmagnetic, and they are generally moreresistant to oxidation and to corrosion in aqueous acid media. Theypossess high hardness, like the other cemented carbide alloys. However,the chromium carbidenickel alloy systems heretofore available havedisplayed a very serious deficiency relative to the other cementedcarbides that has considerably restricted their usage. They aresignificantly lower than the other cemented carbide systems in strengthor specifically transverse rupture strength.

3,445,203 Patented May 20, 1969 Accordingly, it is an object of thisinvention to provide a cemented carbide alloy of high hardness, havinggood corrosion and oxidation resistance, comparatively low density,comparatively high coefficient of thermal expansion, which isnonmagnetic and which, in addition, possesses high strength. Morespecifically, it is an object of this invention to provide a chromiumcarbide-nickel base alloy combining high hardness with high strength.-It is an additional object of this invention to provide a process forpreparing such an alloy.

' I have found that the foregoing and other objects of this inventionmay be achieved by the addition of certain small but critical amounts ofphosphorus to the composition from which the cemented carbide alloy isprepared. More specifically, the addition of amounts ranging from atrace to about 0.4% by weight, based on the total weight of thecomposition, of phosphorus to a chromium carbidenickel powder mixtureproduces a cemented carbide alloy having a strength which is enhanced asmuch as 100% over a comparable chromium carbide-nickel compositionprepared without the phosphorus addition. The alloys of the inventionare prepared by pressing a mixture of the aforementioned powders andsintering the pressed mixture into a compact. The phosphorus ispreferably added to the starting mixture in the form of anickel-phosphorus alloy in amounts adjusted so as to contain thenecessary phosphorus quantity.

The compositions of this invention contain chromium carbide essentiallyin the form of Cr C and a binder of nickel, the binder ranging fromabout 1, and preferably 3, to about 35% by weight. Other metals may beadded to the nickel binder and specifically up to one-third of thenickel may be replaced by molybdenum or tungsten.

The present compositions have very excellent resistance to oxidationrelative to that of cemented carbide alloys in general. They alsodisplay good resistance to corrosion in media such as aqueous acids.They have low density, approximately one-half that of tungstencarbide-cobalt alloys. They are nonmagnetic unlike the cobalt-containingalloys. They have a high coeflicient of thermal expansion, more nearlythat of steel or about double that of tungsten carbide, titanium carbideand tantalum carbide base alloys. They are of high hardness. Unlike thechromium carbide-nickel base alloys heretofore available, thecompositions of this invention are of high strength, having up toapproximately twice the strength of the chromium carbide-nickel basealloys heretofore available.

Whereas the prior chromium carbide base compositions, being low instrength, were necessarily adjusted compositionally so as to maximizestrength, the compositions of the present invention are suificientlystrong such that compromises may be made between strength and hardness.For example, the tungsten carbideFcobalt alloys vary in tungsten carbidecontent from about to 97% by weight, and in cobalt content from about25% to about 3% by weight. Maximum strength is achieved in alloys ofhigh cobalt content. Maximum hardness is achieved in alloys of hightungsten carbide content but some sacrifice in strength must be made toaccomplish the latter. The improvements provided by the presentinvention are such that similar compromises between strength andhardness can now be made within useful limits in chromium carbide basealloys. Thus, the compositions of the present invention may vary inchromium carbide content from about 65% to about 99% by weight. Strengthis greatest in alloys of high nickel content, being about double that ofthe chromium carbide base alloys heretofore available. Hardness isgreatest in alloys of high chromium carbide content, and these havehardness values as high as about 91 R considerably higher than that ofexisting chromium carbide commercial alloys, which generally run from8789 R even though their strengths are equal.

The addition of phosphorus in the very small amounts herein disclosed isbelieved to alter the metallurgical phenomena occurring during sinteringas well as the microstructure of the resulting alloys. While the trueexplanation of this effect is not known with certainty, it is believedthat the presence of phosphorus improves bonding between the carbidegrains and the nickel matrix and results in some modification in themicrostructure of the alloy.

The desired properties are achieved only within the approximatecompositional limits indicated above. Alloys containing more than about35% nickel, or other nickelbase binder such as nickel-molybdenum ornickeltungsten, do not have the requisite hardness, whereas alloyscontaining less than 1% nickel are low in strength. If more than about0.4% phosphorus by weight is employed, the hardness of the resultingalloys suffers substantially. It is difficult to define in absoluteterms the exact minimum amount of phosphorus that is required. Thestrength improvement has, however, been achieved with extremely smallamounts. On the one hand, it is of course encessary that a definiteamount of phosphorus be present. On the other hand, the few parts permillion that may be present in starting materials of usual purity is notsuflicient. Because of inconsistencies and problems associated with thedispersion of very small amounts, additions of a trace amount of theorder of .02% phos phorus (about .12% nickel-phosphorus alloy of 17%phosphorus content) has been found to be an approximate minimum.

The carbon content of the starting chromium carbide Cr C should normallybe between 13.0% and 13.3% by weight, preferably between 13.2% and13.3%. It is believed that the carbon of the chromium carbide powderemployed herein is essentially all combined. Again, this is not knownwith certainty because the analysis of carbon in chromium carbide isvery difiicult and the results subject to some doubt, even utilizing thebest analytical techniques available.

The process of the invention is carried out by first intimately mixingthe chromium carbide powder, nickel metal powder and a source ofphosphorus, the latter preferably being in the form of a mixture of atransition metal such as nickel and phosphorus. A nickel-phosphorusalloy may be made, for example, by reacting ammonium phosphates withnickel metal powders at elevated temperatures, and subsequently crushingand pulverizing the solidified melt. Phosphorus may also be addeddirectly as ammonium phosphate or as an anhydride of phosphorus but theoxygen present in combined form must be carefully expelled by reductionor dissociation prior to sintering. The above-mentioned powders areintimately mixed by ball-milling in a liquid medium such as acetone. Themilled powder is then dried in a protec tive atmosphere, and a smallamount of parafiin is added to facilitate pressing of the powder. Thepressed compacts are heated to about 450 C. in a protective atmosphereto expel the paraffin. The compacts are then sintered in a protectiveatmosphere at about 1200-1300 C.

The following examples illustrate the preparation of compositions inaccordance with the present invention. All parts and percentages are byweight unless otherwise indicated.

Example #1 An alloy containing 17% nickel, 83% chromium carbide, andabout .04% phosphorus was prepared as follows:

Commercially available chromium carbide powder of 13.1% carbon wasadjusted to the desired carbon level in 10 kilo lots. For the purpose,86 grams of chromium powder were added as a carburization promotertogether with 33 grams of carbon (lamp black) such that the carboncontent of the total mix was about 13.3%. These powders were intimatelymixed and heated in carbon boats in a hydrogen atmosphere at atemperature of 1400 to 1500" C. for a period of approximately one hour.The resulting mass was crushed and pulverized to -325 mesh powder.

A nickel-phosphorus alloy was prepared by reacting diammonium phosphateand nickel powder in quantities sufiicient to provide a 17% phosphorusalloy. For this purpose, the mixture was melted in carbon boats in ahydrogen atmosphere at a temperature of approximately 1250 C. The cooledmass was quite friable and easily converted to 200 mesh powder. a

Two hundred and forty-nine grams of the abovementioned chromium carbidepowder, 1 gram of the 17% nickel-phosphorus alloy, and 51 grams ofnickel powder were ball-milled in a 4" cemented tungsten carbide linedball mill containing 250 grams each of A", and A3" cemented tungstencarbide balls, with 200 cc.s of acetone for 24 hours. The powder wasthen dried in a hydrogen atmosphere and 7 /2 grams of parafiin wereadded as a pressing lubricant. The powder was pressed into the desiredshape at 15 t.s.i. pressure and the compacts were presintered in ahydrogen atmosphere at 450 C. The presintered compacts were then placedin a vacuum furnace and heated at 1250 C. for 15 minutes. The resultingpieces had a hardness of 88.5 R 2. density of 7.03 gm./cc., and atransverse rupture strength of 250,000 p.s.i. This is approximatelytwice the strength of heretofore available chromium carbide-nickelalloys.

Example #2 An alloy containing 8% nickel, 92% chromium carbide and about.04% phosphorus was prepared as described above, from 276 grams ofchromium carbide powder, 24 grams of nickel powder, and one gram of thenickelphosphorus alloy. In this case, the sintering temperature was 1275C. This alloy had a hardness of 90.3 R a density of 6.87 gm./cc., and astrength of about 200,000 p.s.i., over 50% greater that that (about120,000 p.s.i.) of heretofore available alloys.

Example #3 An alloy containing 83% chromium carbide, 13.6% nickel, 3.4%molybdenum and about .04% phosphorus was processed as indicated inExample #1, from 249 grams of chromium carbide powder, 40.8 grams ofnickel powder, 10.2 grams of molybdenum powder, and about one gram ofnickel-phosphorus alloy. The sintering temperature in this case was 1275C. This alloy had a hardness of 89.2 R a density of 7.1 gm./cc., and astrength of about 210,000-220,000 p.s.i., about more than the heretoforeavailable alloys.

I claim:

1. A composition for the production of a cemented carbide alloy, saidcomposition consisting essentially of chromium carbide and from 1-35 byweight of a binder containing nickel, said composition containingphosphorus in an amount ranging from a trace to about 0.4% by weightbased on the total weight of the composition.

2. The composition of claim 1 in which the binder is present inpercentages of 3-35 3. The composition of claim 1 in which up toone-third of the total binder by weight is molybdenum or tungsten.

4. The composition of claim 1 in which the chromium carbide containsfrom 13.0 to 13.3% carbon by weight.

5. A hard cemented carbide alloy of enhanced strength consistingessentially of chromium carbide and from 135% by weight of a nickelbinder, said alloy containing consisting essentially of chromium carbidein the form 5 of Cr C and from 15-35% by Weight of a nickel binder, saidalloy containing phosphorus added in an amount ranging from a trace toabout 0.4% by Weight based on the total weight of the alloy.

6 REFERENCES CITED Cemented Carbides, Schwarzkopf & Kieffer, N.Y., TheMacMillan Co., 1960, pp. 177-181.

CARL D. QUARFORTH, Primary Examiner.

ARTHUR I. STEINER, Assistant Examiner.

US. Cl. X.R. 29-1825

